ST. LOUIS (AP) — The St. Louis medical examiner says a fourth person has died from injuries suffered when a boiler exploded at a St. Louis business.

The medical examiner’s office says 53-year-old Clifford Lee died Wednesday at St. Louis University Hospital.

The St. Louis Post-Dispatch reports that Lee was with two friends Monday filling out new employee paperwork at Faultless Linen Company when the boiler exploded at Loy-Lange Box Co. The one-ton boiler flew about 500 feet through the air and crashed through the linen company’s roof.

Lee’s friends, 46-year-old Christopher Watkins and 43-year-old Tonya Suarez-Gonzalez, died at the scene.

A Loy-Lange worker, 59-year-old Kenneth Trentham, also died in the blast.

Federal and local investigators are working to determine the cause of the blast.